Chediak-Higashi syndrome is a primary immune deficiency disease of human (CHS) and mouse (beige, bg). Affected individuals have defective granulocyte and Natural Killer cell activity, pathognomonic giant vesicles in many cell types, and typically die in childhood of infection or malignancy. There is strong evidence that human CHS is homologous to the bg locus in the mouse. Since attempts to identify the genetic defect in CHS directly have been thwarted by absence of chromosomal assignment of CHS or pedigrees adequate for linkage analysis, we have undertaken positional cloning of bg as an antecedent to identification of the CHS gene. Positional cloning is an approach to identifying a disease gene based solely on its chromosomal location, without regard to etiology. Our preliminary experiments have: 1. Localized bg to a 0.24+/-0.17 centiMorgan interval on mouse chromosome 13; Identified a genomic rearrangement within this interval in the SB/LeJ-bg mouse; 3. Characterized 3 yeast artificial chromosomes (YACs) which may span this rearrangement. We propose to identify the defective gene in the bg mouse by examining expressed sequences derived from these YACs for mutations. The human homolog of the bg gene will be cloned and sequenced in CHS patients and normal individuals to identify CHS mutations. Isolation of the CHS gene will permit prenatal genetic diagnosis, enabling institution of treatment for CHS in infancy. Identification of the bg gene will facilitate understanding of mechanisms of regulation of protein trafficking to lysosomes, and of the contribution of vesicular sorting to immune functions such as cellular cytotoxicity. Given the selective impairment of certain immune responses evident in bg mice, identification of the molecular basis of this disease may suggest a novel approach to immune modulation, and imply therapies for CHS which are not currently being considered, and will provide a starting point for gene therapy of CHS.